A thermostat is interposed in the connecting portion between a cooling water channel of an engine body and an inlet housing or an outlet housing connected to a radiator for controlling the temperature of a cooling water circulating in a water jacket (cooling water channel) in an internal combustion engine. The thermostat is designed such that a valve is actuated by the temperature of the cooling water in the cooling water channel to automatically control the flow of the cooling water from the radiator into the cooling water channel. Such a thermostat is disclosed in the following patent document Japanese Patent No. 2640188 and Japanese patent document Japanese Patent No. 3485629. According to these thermostats, a thermo-wax of which phase is changed from solid (crystal) to liquid following the change of water temperature is enclosed in a valve casing (actuator) and a rod is elongated and contracted by the expansion-contraction of the thermo-wax by the phase change, thereby converting the movement of the rod into the valve operation to open and close the channel of the cooling water.
The above-mentioned thermostat is assembled as a single member and is incorporated into the connecting portion between the inlet housing or the outlet housing and the engine body as it is. Or a constituting member of the thermostat is assembled into the inlet housing or the outlet housing and the thermostat may be constructed in the inlet housing or in the outlet housing. In these days, assembling the internal combustion engine, members required for the parts constituting the engine are incorporated in advance and are assembled at the final assembling stage of the engine. Therefore, it is required to provide a system in the inlet housing or the outlet housing in which required members such as thermostat are incorporated in advance, and packed and carried to be brought in the final assembling stage of the engine. The thermostat disclosed in the Japanese patent documents is capable of assembling into the inlet housing or the outlet housing in advance, so that it can satisfy the above-mentioned needs.